This study aims to answer the question: "Does a policy of CEA-prompted second-look surgery following 'curative' resection of colo-rectal cancer produce a decrease in morbidity and mortality due to tumour recurrence, despite sequelae of second-look surgery?" We feel that a definitive answer can only be produced by comparing a CEA-based management regime with one in which CEA results are not consulted for treatment decisions. Therefore, we propose a multicentre, randomised, controlled clinical trial involving 1500 patients recruited over three years and all followed for five years. In the 750 patients in the 'CEA-Aggressive' arm, regular CEA assays will be used to try to detect recurrent tumour while the patient is still asymptomatic; after a significant CEA rise has been recorded, and screening has excluded widespread disease or a non-malignant cause for the CEA rise, second-look surgery to locate and remove any treatable recurrence will be mandatory. In the 750 'Conventional' arm patients, clinical criteria alone will be used by surgeons to detect recurrence and initiate treatment. Surgeons will not be informed of randomisation. All patients in both arms will be monitored in identical fashion, both clinically and by CEA assay; the assay, however, will be performed centrally and the surgeons not informed of results unless: i) a significant rise is recorded and then only if ii) the patient is in the CEA Aggressive arm. Thus, CEA results will never be available to surgeons for patients in the 'Conventional' arm, or in 'CEA Aggressive' arm patients unless a significant rise has occurred. It is hoped that such a procedure will prevent bias in clinical follow up. The Trial will be coordinated from the CRC Trials Centre at King's College Hospital, London. CEA assays will be performed using a radioimmunoassay technique in a single centre at Charing Cross Hospital, London. The study will involve close liaison between surgeons, medical oncologists, biochemists, and data managers. The Trial will produce the following: i) a definitive answer to the above question, ii) an accurate picture of 'lead time' produced by CEA compared to clinical data (available from analysis of the 'Conventional' arm, iii) further data relating CEA levels to tumour histology and topography, iv) a large database on the natural history of bowel cancer.